The catalytic hydrotreatment of hydrocarbon feedstock to remove impurities such as sulfur, nitrogen, and metal compounds is a commonly used process to improve or upgrade such hydrocarbon feedstock. This treatment to remove sulfur and nitrogen from heavy hydrocarbon feedstock is necessary due to various environmental regulations implemented by the United States and other countries. For example, the maximum sulfur concentration in on-road diesel is 15 parts per million (ppm) in the United States. Other organizations are pushing for limits as low as 5 to 10 ppm sulfur in diesel.
In a typical hydrotreating process, the hydrocarbon feedstock is contacted with a hydrotreating catalyst in the presence of hydrogen under process conditions that provide for a treated hydrocarbon product. The hydrotreating catalysts used in these processes generally are composed of an active phase that can include a component from the Group 6 metals, e.g. molybdenum (Mo) or tungsten (W), and a component from either the Group 9 metals, e.g. cobalt (Co), or the Group 10 metals, e.g. nickel (Ni), or a combination thereof, supported on a porous, refractory inorganic oxide material. The references herein to the elements by grouping within the periodic table are as they are listed and defined by the International Union of Pure and Applied Chemistry (IUPAC) Periodic Table of the Elements.
The hydrotreatment of heavy hydrocarbon feedstock is particularly difficult; because, such feeds tend to have high concentrations of contaminating sulfur and nitrogen compounds and may require the use of more severe process conditions than those needed to treat lighter hydrocarbon feedstock. As the quality of feedstock declines, the reaction conditions required to achieve a desired level of hydrotreatment tend to become more severe (e.g., increased temperatures or pressures). This increases production costs and causes more rapid depletion of catalyst activity.
There is a continuing need for improving catalyst performance to offset the decreasing quality of feedstock and the increased processing costs associated therewith. In particular, the ability of a catalyst to achieve acceptable sulfur and nitrogen removal at lower temperatures is quite valuable; because, lower temperatures require less energy input which directly reduces production costs.
One catalyst found to be useful in the hydroprocessing of heavy hydrocarbon feedstocks is disclosed in U.S. Pat. No. 4,738,944 (Robinson et al.). The catalyst disclosed in this patent contains nickel, phosphorus and molybdenum supported on alumina, and it contains up to about 10, usually from 1 to 8 percent, and preferably from 2 to 6 percent by weight of nickel metal components, calculated as the monoxide. The catalyst also contains from about 16 to about 23 and preferably from 19 to 21.5 percent by weight molybdenum metal components, calculated as molybdenum trioxide (MoO3). The pore structure of the catalyst is such that it has a narrow pore size distribution with at least about 75 percent, preferably at least about 80 percent, and most preferably at least about 85 percent of the total pore volume in pores of diameter from about 50 to about 110 angstroms. Ordinarily, the catalyst has less than about 10 percent of its total pore volume in pores of diameter below about 50 angstroms.
Another hydroprocessing catalyst is disclosed in U.S. Pat. No. 7,824,541 (Bhan) that is particularly useful in the treatment of distillate feedstocks to manufacture low-sulfur distillate products. This catalyst is a co-mulled mixture of molybdenum trioxide, a Group VIII metal compound, and an inorganic oxide material. The co-mulled mixture is calcined. The molybdenum content of the catalyst is in the range of from 10.5 to 33 wt. %, calculated as an oxide. If the Group VIII metal component is nickel, it is present in the catalyst in the range of from 3.8 to 15.3 wt. %, calculated as an oxide. The catalyst also has a mean pore diameter that is in a specific and narrow range of from 50 to 100 angstroms. There is less than 4.5 percent of the total pore volume that is contained in its macropores having pore diameters greater than 350 angstroms and less than 1 percent of the total pore volume contained in its macropores having pore diameters greater than 1000 angstroms.
Disclosed in U.S. Pat. No. 7,871,513 (Bhan) is a catalyst that is useful in the hydroprocessing of heavy hydrocarbon feedstocks. This catalyst is a calcined mixture made by calcination of a formed particle of a mixture comprising molybdenum trioxide, a nickel compound, and an inorganic oxide material. The molybdenum content of the catalyst is in the range upwardly to 18 wt. %, calculated as an oxide. The nickel content of the catalyst is in the range upwardly to 5.1 wt. %, calculated as an oxide. The molybdenum source used in the preparation of the catalyst is in the form of molybdenum trioxide that is in a finely divided state.
While the catalysts described above have been shown to have good hydroprocessing activity, there are continuing efforts to find new or improved catalyst compositions having increased catalytic activity or improved stability, or both. Any improvement in catalyst activity can result in the lowering of required reactor temperatures to obtain a product with reduced concentrations of nitrogen or sulfur as compared to a feedstock that is contaminated with these components. The lower reactor temperatures provide for energy savings and will extend the life of a catalyst. There also are ongoing efforts to find more economical methods of manufacturing the catalyst compositions.